Despite considerable advances toward understanding the molecular basis of calcium and phosphorus transfer across the intestine and other epithelial membranes, much is yet to be revealed. This also pertains to the molecular basis of vitamin D action. The proposed investigations represent a continuing effort to uncover new aspects of the problem and to aid in defining the mechanisms involved, which should find relevance to specific human and animal disease states. Detailed investigations on the properties (including both amino acid sequence and X-ray crystallographic studies), cellular and subcellular localization, distribution and function of the vitamin D-induced, calcium-binding protein (CaBP) and other vitamin D-dependent molecules will continue. Regulatory mechanisms will continue to be investigated, as will the use of CaBP as a sensitive indicator of vit. D activity. The organ culture system with embryonic intestine, which responds to vitamin D and metabolites, constitutes a unique tool for the investigation of specific aspects of the vit. D-dependent absorptive process. Using the organ cultured embryonic intestine, the relationships between the adenylate cyclase system, vitamin D, hydrocortisone and the calcium ion (and other aspects) will be studied. Establishment of a radioimmunoassay for bovine and chick CaBPs will be undertaken. Further purification and characterization of the 1,25(OH)2D3-like factors present in calcinogenic plants will be continued. Supportive evidence for a shunt path for calcium across the intestine will be sought. Detailed investigations on the physiological and molecular basis of phosphate absorption are planned. The potential applicatons of these findings to pertinent disease states in humans and animals will be continually surveyed.